Add documentation for KStackWatch under Documentation/. It provides an overview, main features, usage details, configuration parameters, and example scenarios with test cases. The document also explains how to locate function offsets and interpret logs.
Signed-off-by: Jinchao Wang <[email protected]> --- Documentation/dev-tools/index.rst | 1 + Documentation/dev-tools/kstackwatch.rst | 377 ++++++++++++++++++++++++ 2 files changed, 378 insertions(+) create mode 100644 Documentation/dev-tools/kstackwatch.rst diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst index 4b8425e348ab..272ae9b76863 100644 --- a/Documentation/dev-tools/index.rst +++ b/Documentation/dev-tools/index.rst @@ -32,6 +32,7 @@ Documentation/process/debugging/index.rst lkmm/index kfence kselftest + kstackwatch kunit/index ktap checkuapi diff --git a/Documentation/dev-tools/kstackwatch.rst b/Documentation/dev-tools/kstackwatch.rst new file mode 100644 index 000000000000..9b710b90e512 --- /dev/null +++ b/Documentation/dev-tools/kstackwatch.rst @@ -0,0 +1,377 @@ +.. SPDX-License-Identifier: GPL-2.0 + +================================= +Kernel Stack Watch (KStackWatch) +================================= + +Overview +======== + +KStackWatch is a lightweight debugging tool designed to detect kernel stack +corruption in real time. It installs a hardware breakpoint (watchpoint) at a +function's specified offset using *kprobe.post_handler* and removes it in +*fprobe.exit_handler*. This covers the full execution window and reports +corruption immediately with time, location, and call stack. + +Main features: + +* Immediate and precise stack corruption detection +* Support for multiple concurrent watchpoints with configurable limits +* Lockless design, usable in any context +* Depth filter for recursive calls +* Low overhead of memory and CPU +* Flexible debugfs configuration with key=val syntax +* Architecture support: x86_64 and arm64 +* Auto-canary detection to simplify configuration + +Performance Impact +================== + +Runtime overhead was measured on Intel Core Ultra 5 125H @ 3 GHz running +kernel 6.17, using test4: + ++------------------------+-------------+---------+ +| Type | Time (ns) | Cycles | ++========================+=============+=========+ +| entry with watch | 10892 | 32620 | ++------------------------+-------------+---------+ +| entry without watch | 159 | 466 | ++------------------------+-------------+---------+ +| exit with watch | 12541 | 37556 | ++------------------------+-------------+---------+ +| exit without watch | 124 | 369 | ++------------------------+-------------+---------+ + +From a broader perspective, the overall comparison is as follows: + ++----------------------------+----------------------+-------------------------+ +| Mode | CPU Overhead (add) | Memory Overhead (add) | ++============================+======================+=========================+ +| Compiled but not enabled | None | ~20 B per task | ++----------------------------+----------------------+-------------------------+ +| Enabled, no function hit | None | ~few hundred B | ++----------------------------+----------------------+-------------------------+ +| Func hit, HWBP not toggled | ~140 ns per call | None | ++----------------------------+----------------------+-------------------------+ +| Func hit, HWBP toggled | ~11–12 µs per call | None | ++----------------------------+----------------------+-------------------------+ + +The overhead is minimal, making KStackWatch suitable for production +environments where stack corruption is suspected but kernel rebuilds are not +feasible. + +Kconfig Options +=============== + +The following configuration options control KStackWatch builds: + +- CONFIG_KSTACKWATCH + + Builds the kernel with KStackWatch enabled. + +- CONFIG_KSTACKWATCH_PROFILING + + Measures probe runtime overhead for performance analysis and tuning. + +- CONFIG_KSTACKWATCH_TEST + + Builds a test module to validate KStackWatch functionality. + +Usage +===== + +KStackWatch provides optional configurations for different use cases. +CONFIG_KSTACKWATCH enables real-time stack corruption detection using hardware breakpoints and probes. +CONFIG_KSTACKWATCH_PROFILING allows measurement of probe latency and overhead for performance analysis. +CONFIG_KSTACKWATCH_TEST builds a test module for validating KStackWatch functionality under controlled conditions. + +KStackWatch is configured through */sys/kernel/debug/kstackwatch/config* using a +key=value format. Both long and short forms are supported. Writing an empty +string disables the watch. + +.. code-block:: bash + + # long form + echo func_name=? func_offset=? ... > /sys/kernel/debug/kstackwatch/config + + # short form + echo fn=? fo=? ... > /sys/kernel/debug/kstackwatch/config + + # disable + echo > /sys/kernel/debug/kstackwatch/config + +The func_name and the func_offset where the watchpoint should be placed must be +known. This information can be obtained from *objdump* or other tools. + +Required parameters +-------------------- + ++--------------+--------+-----------------------------------------+ +| Parameter | Short | Description | ++==============+========+=========================================+ +| func_name | fn | Name of the target function | ++--------------+--------+-----------------------------------------+ +| func_offset | fo | Instruction pointer offset | ++--------------+--------+-----------------------------------------+ + +Optional parameters +-------------------- + +Default 0 and can be omitted. +Both decimal and hexadecimal are supported. + ++--------------+--------+------------------------------------------------+ +| Parameter | Short | Description | ++==============+========+================================================+ +| auto_canary | ac | Automatically calculated canary sp_offset | ++--------------+--------+------------------------------------------------+ +| depth | dp | Recursion depth filter | ++--------------+--------+------------------------------------------------+ +| | | Maximum number of concurrent watchpoints | +| max_watch | mw | (default 0, capped by available hardware | +| | | breakpoints) | ++--------------+--------+------------------------------------------------+ +| panic_hit | ph | Panic system on watchpoint hit (default 0) | ++--------------+--------+------------------------------------------------+ +| sp_offset | so | Watching addr offset from stack pointer | ++--------------+--------+------------------------------------------------+ +| watch_len | wl | Watch length in bytes (1, 2, 4, 8 onX86_64) | ++--------------+--------+------------------------------------------------+ + + +Workflow Example +================ + +Silent corruption +----------------- + +Consider *test3* in *kstackwatch_test.sh*. Run it directly: + +.. code-block:: bash + + echo test3 >/sys/kernel/debug/kstackwatch/test + +Sometimes, *test_mthread_victim()* may report as unhappy: + +.. code-block:: bash + + [ 7.807082] kstackwatch_test: victim[0][11]: unhappy buf[8]=0xabcdabcd + +Its source code is: + +.. code-block:: c + + static void test_mthread_victim(int thread_id, int seq_id, u64 start_ns) + { + ulong buf[BUFFER_SIZE]; + + for (int j = 0; j < BUFFER_SIZE; j++) + buf[j] = 0xdeadbeef + seq_id; + + if (start_ns) + silent_wait_us(start_ns, VICTIM_MINIOR_WAIT_NS); + + for (int j = 0; j < BUFFER_SIZE; j++) { + if (buf[j] != (0xdeadbeef + seq_id)) { + pr_warn("victim[%d][%d]: unhappy buf[%d]=0x%lx\n", + thread_id, seq_id, j, buf[j]); + return; + } + } + + pr_info("victim[%d][%d]: happy\n", thread_id, seq_id); + } + +From the source code, the report indicates buf[8] was unexpectedly modified, +a case of silent corruption. + +Configuration +------------- + +Since buf[8] is the corrupted variable, the following configuration shows +how to use KStackWatch to detect its corruption. + +func_name +~~~~~~~~~~~ + +As seen, buf[8] is initialized and modified in *test_mthread_victim*\(\) , +which sets *func_name*. + +func_offset & sp_offset +~~~~~~~~~~~~~~~~~~~~~~~~~ +The watchpoint should be set after the assignment and as close as +possible, which sets *func_offset*. + +The watchpoint should be set to watch buf[8], which sets *sp_offset*. + +Use the objdump output to disassemble the function: + +.. code-block:: bash + + objdump -S --disassemble=test_mthread_victim vmlinux + +A shortened output is: + +.. code-block:: text + + static void test_mthread_victim(int thread_id, int seq_id, u64 start_ns) + { + ffffffff815cb4e0: e8 5b 9b ca ff call ffffffff81275040 <__fentry__> + ffffffff815cb4e5: 55 push %rbp + ffffffff815cb4e6: 53 push %rbx + ffffffff815cb4e7: 48 81 ec 08 01 00 00 sub $0x108,%rsp + ffffffff815cb4ee: 89 fd mov %edi,%ebp + ffffffff815cb4f0: 89 f3 mov %esi,%ebx + ffffffff815cb4f2: 49 89 d0 mov %rdx,%r8 + ffffffff815cb4f5: 65 48 8b 05 0b cb 80 mov %gs:0x280cb0b(%rip),%rax # ffffffff83dd8008 <__stack_chk_guard> + ffffffff815cb4fc: 02 + ffffffff815cb4fd: 48 89 84 24 00 01 00 mov %rax,0x100(%rsp) + ffffffff815cb504: 00 + ffffffff815cb505: 31 c0 xor %eax,%eax + ulong buf[BUFFER_SIZE]; + ffffffff815cb507: 48 89 e2 mov %rsp,%rdx + ffffffff815cb50a: b9 20 00 00 00 mov $0x20,%ecx + ffffffff815cb50f: 48 89 d7 mov %rdx,%rdi + ffffffff815cb512: f3 48 ab rep stos %rax,%es:(%rdi) + + for (int j = 0; j < BUFFER_SIZE; j++) + ffffffff815cb515: eb 10 jmp ffffffff815cb527 <test_mthread_victim+0x47> + buf[j] = 0xdeadbeef + seq_id; + ffffffff815cb517: 8d 93 ef be ad de lea -0x21524111(%rbx),%edx + ffffffff815cb51d: 48 63 c8 movslq %eax,%rcx + ffffffff815cb520: 48 89 14 cc mov %rdx,(%rsp,%rcx,8) + ffffffff815cb524: 83 c0 01 add $0x1,%eax + ffffffff815cb527: 83 f8 1f cmp $0x1f,%eax + ffffffff815cb52a: 7e eb jle ffffffff815cb517 <test_mthread_victim+0x37> + if (start_ns) + ffffffff815cb52c: 4d 85 c0 test %r8,%r8 + ffffffff815cb52f: 75 21 jne ffffffff815cb552 <test_mthread_victim+0x72> + silent_wait_us(start_ns, VICTIM_MINIOR_WAIT_NS); + ... + ffffffff815cb571: 48 8b 84 24 00 01 00 mov 0x100(%rsp),%rax + ffffffff815cb579: 65 48 2b 05 87 ca 80 sub %gs:0x280ca87(%rip),%rax # ffffffff83dd8008 <__stack_chk_guard> + ... + ffffffff815cb5a1: eb ce jmp ffffffff815cb571 <test_mthread_victim+0x91> + } + ffffffff815cb5a3: e8 d8 86 f1 00 call ffffffff824e3c80 <__stack_chk_fail> + + +func_offset +^^^^^^^^^^^ + +The function begins at ffffffff815cb4e0. The *buf* array is initialized in a loop. +The instruction storing values into the array is at ffffffff815cb520, and the +first instruction after the loop is at ffffffff815cb52c. + +Because KStackWatch uses *kprobe.post_handler*, the watchpoint can be +set right after ffffffff815cb520. However, this will cause false positive +because the watchpoint is active before buf[8] is assigned. + +An alternative is to place the watchpoint at ffffffff815cb52c, right +after the loop. This avoids false positives but leaves a small window +for false negatives. + +In this document, ffffffff815cb52c is chosen for cleaner logs. If false +negatives are suspected, repeat the test to catch the corruption. + +The required offset is calculated from the function start: + +*func_offset* is 0x4c (ffffffff815cb52c - ffffffff815cb4e0). + +sp_offset +^^^^^^^^^^^ + +From the disassembly, the buf array is at the top of the stack, +meaning buf == rsp. Therefore, buf[8] sits at rsp + 8 * sizeof(ulong) = +rsp + 64. Thus, *sp_offset* is 64. + +Other parameters +~~~~~~~~~~~~~~~~~~ + +* *depth* is 0, as test_mthread_victim is not recursive +* *max_watch* is 0 to use all available hwbps +* *watch_len* is 8, the size of a ulong on x86_64 + +Parameters with a value of 0 can be omitted as defaults. + +Configure the watch: + +.. code-block:: bash + + echo "fn=test_mthread_victim fo=0x4c so=64 wl=8" > /sys/kernel/debug/kstackwatch/config + +Now rerun the test: + +.. code-block:: bash + + echo test3 >/sys/kernel/debug/kstackwatch/test + +The dmesg log shows: + +.. code-block:: text + + [ 7.607074] kstackwatch: ========== KStackWatch: Caught stack corruption ======= + [ 7.607077] kstackwatch: config fn=test_mthread_victim fo=0x4c so=64 wl=8 + [ 7.607080] CPU: 2 UID: 0 PID: 347 Comm: corrupting Not tainted 6.17.0-rc7-00022-g90270f3db80a-dirty #509 PREEMPT(voluntary) + [ 7.607083] Call Trace: + [ 7.607084] <#DB> + [ 7.607085] dump_stack_lvl+0x66/0xa0 + [ 7.607091] ksw_watch_handler.part.0+0x2b/0x60 + [ 7.607094] ksw_watch_handler+0xba/0xd0 + [ 7.607095] ? test_mthread_corrupting+0x48/0xd0 + [ 7.607097] ? kthread+0x10d/0x210 + [ 7.607099] ? ret_from_fork+0x187/0x1e0 + [ 7.607102] ? ret_from_fork_asm+0x1a/0x30 + [ 7.607105] __perf_event_overflow+0x154/0x570 + [ 7.607108] perf_bp_event+0xb4/0xc0 + [ 7.607112] ? look_up_lock_class+0x59/0x150 + [ 7.607115] hw_breakpoint_exceptions_notify+0xf7/0x110 + [ 7.607117] notifier_call_chain+0x44/0x110 + [ 7.607119] atomic_notifier_call_chain+0x5f/0x110 + [ 7.607121] notify_die+0x4c/0xb0 + [ 7.607123] exc_debug_kernel+0xaf/0x170 + [ 7.607126] asm_exc_debug+0x1e/0x40 + [ 7.607127] RIP: 0010:test_mthread_corrupting+0x48/0xd0 + [ 7.607129] Code: c7 80 0a 24 83 e8 48 f1 f1 00 48 85 c0 74 dd eb 30 bb 00 00 00 00 eb 59 48 63 c2 48 c1 e0 03 48 03 03 be cd ab cd ab 48 89 30 <83> c2 01 b8 20 00 00 00 29 c8 39 d0 7f e0 48 8d 7b 10 e8 d1 86 d4 + [ 7.607130] RSP: 0018:ffffc90000acfee0 EFLAGS: 00000286 + [ 7.607132] RAX: ffffc90000a13de8 RBX: ffff888102d57580 RCX: 0000000000000008 + [ 7.607132] RDX: 0000000000000008 RSI: 00000000abcdabcd RDI: ffffc90000acfe00 + [ 7.607133] RBP: ffff8881085bc800 R08: 0000000000000001 R09: 0000000000000000 + [ 7.607133] R10: 0000000000000001 R11: 0000000000000000 R12: ffff888105398000 + [ 7.607134] R13: ffff8881085bc800 R14: ffffffff815cb660 R15: 0000000000000000 + [ 7.607134] ? __pfx_test_mthread_corrupting+0x10/0x10 + [ 7.607137] </#DB> + [ 7.607138] <TASK> + [ 7.607138] kthread+0x10d/0x210 + [ 7.607140] ? __pfx_kthread+0x10/0x10 + [ 7.607141] ret_from_fork+0x187/0x1e0 + [ 7.607143] ? __pfx_kthread+0x10/0x10 + [ 7.607144] ret_from_fork_asm+0x1a/0x30 + [ 7.607147] </TASK> + [ 7.607147] kstackwatch: =================== KStackWatch End =================== + [ 7.807082] kstackwatch_test: victim[0][11]: unhappy buf[8]=0xabcdabcd + +The line ``RIP: 0010:test_mthread_corrupting+0x48/0xd0`` shows the exact +location where the corruption occurred. Now that the ``corrupting()`` function has +been identified, it is straightforward to trace back to ``buggy()`` and fix the bug. + + +More usage examples and corruption scenarios are provided in +``kstackwatch_test.sh`` and ``mm/kstackwatch/test.c``. + +Limitations +=========== + +* Limited by available hardware breakpoints +* Only one function can be watched at a time +* Canary search limited to 128 * sizeof(ulong) from the current stack + pointer. This is sufficient for most cases, but has three limitations: + + - If the stack frame is larger, the search may fail. + - If the function does not have a canary, the search may fail. + - If stack memory occasionally contains the same value as the canary, + it may be incorrectly matched. + + In these cases, the user can provide the canary location using + ``sp_offset``, or treat any memory in the function prologue + as the canary. -- 2.43.0
